Conventionally, an alarming device this mounted in a subject vehicle and generates an alarm relating to an obstacle based on an expectation result is proposed (e.g., JP-H11-139229 A). In this alarming device, position data of an obstacle includes a relative distance and relative orientation between the subject vehicle and the obstacle, and a size of the object and is memorized. In consideration of the preceding behavior of the obstacle using its position data, the subsequent behavior of the obstacle is expected. An alarm relating to the obstacle is then generated based on the expectation result.
The above alarming device obtains position data every a single obstacle even when multiple obstacles are present. When many obstacles are present, it is supposed that obtaining the position data relating to many obstacles needs a long time. In this case, an alarm is possibly not generated at a proper timing.